Tanning



Patented Mar. 17, 1942 TANNING Joseph s. Kirk, Seven Hills, Ohio, assignor to n. I. du Pont de'Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 6, 1940,

Serial No. 355,606

11 Claims.

This invention relates to the production of leather and it is more particularly directed to compositions and processes for the tanning of skins with silicic acid sols containing an amide.

The conversion of skins into leather effects profound changes in their physical characteristics. They become opaque and a distinct change in structure becomes apparent. The stiffness characteristic of dry rawhides disappears to a great extent and they become relatively flexible. They assume a characteristic attractive appearance and soft feel. Tanning also renders the product imputrescible. For useful tanning these changes must be relativeiypermanent and the leather should be resistant to de-tanning by water, at least at ordinary temperatures.

The ancient and well-developed art of tanning customarily effects the conversion of skins into leather by the use of vegetable preparations that contain tannins. Vegetable tannin materials, such as quebracho extract, are quite slow in their action, and tanning processes employing them require excessively long periods of time for completion. The leather produced using vegetable tanning materials is characteristically colored, and it is difficult to obtain good true colors upon dyeing. It is particularly difficult to produce satisfactory light-colored leather.

Tanning processes employing metal compounds, such as chromium sulfate, are also known. Such processes are, however, subject to a number of disadvantages in operation, and the leather produced is characteristically colored. White leather, so-called, can be produced by employing bleaching but this leather is not truly White for it has a distinct blue, yellow, green, or gray tint.

silicic acid sols have been proposed as tanning agents but their use has been none too successful.

It has been proposed to add various metal compounds such as aluminum or iron salts. Such tanning agents including silica sols show so great an afiinity for skin substance that over-tanning results. The leather produced is harsh, stiff, and of a generally unsatisfactory character. Changes occur in leather tanned according to the processses proposed in the art and the leather soon becomes even stiffer, sometimes to the point. of brittleness.

It has further been proposed to modify socalled silica sol tanning agents by the inclusion of various organic compounds such as lactic acid 1 and sulfite cellulose waste, but the addition of such modifying agents has not effected sufficient improvement in the tannages to make them entirely satisfactory. The leather produced is still of poor character and has only a poor quality compared with vegetable and chrome tans which have found acceptance in 'the trade.

. It is an object of this invention to provide novel tanning processes and compositions. It is a further object to provide tanning compositions and processes which produce a good quality of leather at relatively low cost. It is a, further object of this invention to provide compositions and processes employing silica sols in the-tanning of skins to produce leather of good character. It is a still further object to provide processes and compositions which permit rapid, simple, and easily controlled tanning of skins. It is a still further object of this invention to improve upon prior art processes employing silicic acid sols for the tanning of skins. It is still another object of this invention to produce white leather. Still further objects will become apparent hereinafter.

These and other objects of my invention are attained by the use of an amide with a silicic acid sol. According to my invention an amide may be used with silicic acid sols which contain metal compounds and other similar modifying agents and in its preferred embodiment my invention contemplates the use of silicic acid sols containing a soluble aluminum salt and a urea. Leather produced according to the preferred processes of this invention is of excellent character, being opaque, soft, full, and flexible.

The silicic acid sols used as tanning agents according to this invention are ordinarily produced by lowering the pH of a solution of a soluble silicate. When first formed the sols probably contain considerable amounts of silicic acid but silicic acid is probably present as such only for a relatively short time and then conversion to silica begins. Most silicic acid sols are, as is well-known, to an extent unstable and after a period of time varying from a few hours and upwards change over to a silica gel.

As has been the practice heretofore, silicic sols should be used as tanning solutions before gelling occurs and before they become too stringent. It will generally be preferable to use these silicic acid sols shortly after they are made up even though sols modified with amides according to this invention are of increased stability.

Silicic acid sols for use as tanning agents according to this invention may be prepared in various other ways already known to the art. It will ordinarily be desirable to start with a soluble silicate such as sodium or potassium silicate and reduce the pH of the silicate by adding it to an tain acidic metal salts themselves have an effect in the resulting product. There may be used for reducing the pH of a silicate such acidic materials as sulfuric acid, sulfamic acid, hydrochloric acid, nitric acid, thionic acids, acetic acid, lactic acid, and other similar inorganic and organic acids. Acidulous salts may similarly be used and there may be employed, for instance, sodium bisulfate. monosodium phosphate, acid tartrates, zinc chloride, titanium sulfate, aluminum sulfate, and chromium sulfate.

It willordinarily be desirable to reduce the pH of a silicic acid sol to about 1.5 to 3.5. Still more specifically as will be pointed out hereinafter, it will be preferred to reduce the pH to about 2.0 to 3.0. It will be understood that while any acidulous material may be used to increase the acidity of silicate solutions to produce silica sols, some acidulous materials. do not themselves have a low enough pH to produce the final acidity desired. In this event such an acidulous mate-.

rial may be used in combination with a. more acidulous material, and as will be evident mixtures of two or more acidulous materials may be used as seems desirable.

There may be included in the silicic acid so various metal compounds. If an acidulous metal compound is selected it may serve to supply a part or all of the acidity needed to produce the sol. There may be included in silicic acid sols according to this invention such metal cations as aluminum, chromium, iron, zinc, and titanium. These metals may be added in the form of any of their soluble compounds and there may be used, for instance, aluminum sulfate, aluminum formate, aluminum lactate, aluminum chloride, chromium sulfate, titanium sulfate, and iron sulfate.

The preferred tanning compositions according to the present invention are comprised of silicic acid sols containing a soluble aluminum compound. Aluminum is preferably introduced into a silicic acid sol in the form of aluminum sulfate, due consideration being given to the acidity 'AlzOstSiOz ratio lies between the limits of 1:2

to 1:4 parts by weight. With much smaller ratios than 1:2 excessivev puckering of the leather takes place and above about the 1:4 ratio the leather tends to become fiat and tinney. The ratio of AlzOazSiOzcan be obtained, as has been indicated above, by the use of various silicates and aluminum compounds withvarious acidulous materials as required. 7

The concentration of silicic acid sols may be selected as in prior processes with this sol. made up for use may contain about four per cent SiOz and two per cent A1203 by weight. I have used six per centSiOz and three per cent A1203. Upon dilution to make a tanning bath it will seldom be found desirable to have an S10:

' content in excess of about two per cent.

Sols

conditions when approximately two mols of urea- According to the present invention silicic acid sols are modified by the inclusion of an amide. The amount of amide used should in each instance be about that required to obtain an optimum effect upon the tannage. If too much amide is used the tannage may be slowed down unduly and with still greater quantities of amide may be inhibited entirely. With too small an amountof amide the benefits of the invention will not be realized in the fullest measure and the tannage may be too rapid and astringent.

It will in general be preferred to use amides which are N-substituted and the more substituents the greater the effectiveness of the amide in stabilizing the silicic acid sols and in diminishing their rate of tanning. It will be understood that amides should not contain groups which will react with and precipitate the silicic acid sol or tanning assistants which are to be used. It will further be understood that the amides should be at least slightly soluble so that they may exercise their beneficial functions in the sols. As typical amides there may be tabulated the following:

- A preferred group of amides for inclusion in' silicic acid sols are the ureas. The following may be given as typical ureas.

1. Urea 2. Thio urea 3. Tetramethyl urea 4. Tetraethyl urea 5. Dimorphilide urea 0f the ureas, urea itself is from many standpoints among the most desirable for addition to silicic acid sols. Urea is relatively low in cost compared to the substituted ureas and it exercises a number of uniquely beneficial effects upon leather.

The amount of urea to use in a particular case like the amount of amides varies considerably with the result desired and with the particular silicic acid sol and the particular tanning assistants chosen. In general, it may be indicated that from about one to eight mols of urea should be used for each mol of SiOz in the sol. The amount of urea should of course not be so great as to slow the tannage down unduly and should be suflicient to prevent too rapid a tanning. The most satisfactory results are obtained for average are used for each mol of SiOz.

The silicic acid sol tanning agents of this invention may be used for the treatment of any type of skin adapted to be converted into leather or fur. It is to be observed that the term skin" is used generically to include heavy skins such as cowhide, lighter skins such as goatskin and calfskin, and also skins of fur-bearing animals.

Itwill naturally be necessary to adapt the proc esses to the type of skin to be treated. For instance, goatskin may be more rapidly tanned than calfskin, but those skilled in the art are well aware of such factors and may readily adapt thecompositions of this invention to the specific needsof particular specialized problems.

Skins to be tanned according to the processes of this invention may be prepared in any of the manners customary in the art. They may suitably be unhaired, limed, delimed, and bated and pickled. The skins may if desired be de-pickled. They may also be given a salt liquor treatment to soften them up and to put them in a more uniform condition as is the practice in prior art tanning processes.

While skins prepared in any manner may be treated according to the processes of this invention, the pH of the skins is important. As will be observed below, the tanning is effected at certain conditions of acidity and the skins may tend to change these conditions. The tanning solution may suitably be controlled to compen sate for variance of the pH of the skins from that of the tanning solution. It may sometimes be found advantageous to adapt the processes for preparing the skins so that a skin prepared for tanning is approximately at the pH of the tanning solution.

Too rapid reaction of the tanning agent with the skins in the first stages of the process should be avoided in order to prevent surface hardening of the skin. The amount of urea used should,

as indicated above, be such as to assist in preventing too rapid a tanning and the initial pH of the solution should not be too high. The initial pH of the solution will ordinarily be from about 1.5 to 3.0. More specifically, it will be found preferable to use a pH of about 2.5. If the initial pH is much above 3.5 the tanning will proceed too rapidly in th initial stages though it will be understood that the pH to be selected in any particular instance will depend upon the specific conditions of the tannage. If too low a pH is used combination of. silica with the skin is too slow and, additionally, excessive acidity may deleteriously affect the skin.

After satisfactory penetration has been obtained the pH of the solution is raised by the addition of any suitable alkali. Raising the pH accelerates the tanning action. The pH should at this point be raised to about 3.5 to 4.0. Sodium carbonate or other alkali metal or ammonium hydroxides or carbonates or organic bases as ethanolamines may be used to raise the pH. 1

The considerations as to the type of apparatus to use, the time to be allowed for tanning, the

temperatures of tanning, and the amounts of tanning agent are substantially those known to the art in connection with prior art tanning processes. The time allowed for tanning must be sufiicient to permit complete penetration of the tanning agent, and the pH should not be raised but unduly large amounts should not be used as they will not be taken up by the leather in a reasonable period of time and they may cause I over-tannage.

The leather obtained according to the preferred processes of this invention using a silicic acid sol with a soluble aluminum compound is white, but it may if desired be treated with white pigment or may be colored during or after the tanning processes by inclusion of any of the common leather dyes and pigments. Leather produced by any of the processes of this invention may similarly be colored and dyed in accordance with the practices known in the art.

Silicic acid s01 tanning agents, including urea. may if desired be used together with other tanning agents or tanning assistants heretofore known to the art. They may, for instance, be used in conjunction with vegetable tanning materials such as quebracho extract, with chrome tans, or with synthetic tanning agents such as those containing sulfonic acid groups. The silica sols may be used to begin tanning and other tanning agents used to finish g ff the leather or, al:

' ternatively, the other tans may be used to begin the tanning and the silicic acid sols used for finishing. When other tanning agents are used simultaneously with the silicic acid sols due consideration must of course be given to the acidity of the resulting solution so that it will fall within the ranges heretofore mentioned.

As has been suggested in the foregoing, the tanning solutions may contain any of the various additions suggested by the art as beneficial in silicic acid sols. Suitable dispersing or stabilizing agents may be used as desired and in general the practices in the art with respect to silica sols may advantageously be employed in specific instances.

The tanning agents of this invention may be offered to the trade as sols containing urea and additionally containing metals and other assistants as desired. If it is likely that the tanning agents will not be used for quite a while it may be found preferable to sell two components, one. for instance, containing sodium silicate and any suitable modifying agents which do not lower the pH to a point so low that a sol is formed, and a second component containing a urea and an acidic material in such amount that when the two components are mixed 8, sol of substantially the character desired will be obtained. Naturally the second component like the first may include various modifying agents and tanning assistants and if a soluble metal compound is to be used it will ordinarily be found preferable to include it in the second component.

Leather produced according to the processes of this invention may be subjected to any of the various kinds of finishing treatments customarily used. The leather,-if desired, may be suitably filled or given any of the numerous surface treatments customary in the art.

My invention will be better understood by reference to the following illustrative examples:

EXAMPLE I A tanning solution according to the present invention was prepared by making up the A and B solutions as tabulated and adding solution B to solution A with agitation. The tanning solution obtained was clear and had a pH of 2.7.

Solution A Parts Aluminum sulfate (A12(SO4)3'18H2O) 32.5 Urea l8 Sulfuric acid (10% H2804) Water 25.5

Solution B Sodium silicate (Nazzsioz ratio 1:3.5

42.5B) 34 Water 91 Goatskins which had been prepared for tanning by the usual procedure of unhairing, deliming, and pickling were tanned with this solution. Fifteen parts by weight of sodium chloride,

250 parts by weight'of the tanning solution prepared as above, and 250 parts by weight of water were charged into a tanning drum and thoroughly mixed. There was then added to the drum 125 parts by weight of goatskins (drained, pickled weight).

The skins were agitated for one hour and then addition of the ten per cent solution of sodium bicarbonate was begun. By repeated additions of sodium bicarbonate over a course of seven and one-half hours the pH of the solution was brought to the final value of pH 4. The solution was then drained from the drum.

The tanned skins were piled down overnight and the next morning they were neutralized to pH 5.5 with a ten per cent sodium bicarbonate solution during one and one-quarter hours further agitation. Subsequently the hides were washed with water and fatliquored with a sulfonated oil neats-foot oil mixture.

EXAMPLE II A tanning solution was prepared according to the present invention by making up the A and B solutions as tabulated and adding solution B to solution A with agitatlons. The tanning solution was clear and had a pH of 2.0.

Solution A Parts 20% Sulfuric acid 8 Watch; l7 Tetra methyl urea 2.9

Solution B Silicate Be. 42.5, ratio 1:3.25 10.1 Water 17.9

Calfskins which had been prepared by the usual procedure of unhairing, deliming and pickling were tanned with this solution. 32 parts of the pickled calfskin were placed in a tanning drum and agitated for five minutes with 32 parts of five per cent solution of sodium chloride to prepare the skins for tannage. drum was added 45 parts of the silica sol prepared above.

The skins were agitated for two hours and let stand overnight inthe liquor.

The following day the skins were agitated for six hours during the course of which they were.

neutralized to pH 4.75 by the addition of small quantities of ten per cent sodium bicarbonate. Total quantity added, 5.5 parts ten per cent sodium bicarbonate.

The skins were piled down over night, after which they were washed, shaved, and fatliquored with a sulfonated oil emulsion. They were then dried, tacked and staked. The leather was smooth, white, and supple.

EXAMPLE III A silicic acid sol was prepared exactly as in Example II except that tetra ethyl urea was used instead of tetra methyl urea. In this case To the tanning 4.3 parts were used in place or tetra methyl urea.

The resulting silicic acid sol was a milky emulsion at pH 2.2.

This sol was prepared using the following components: Solution A Parts 20% Sulfuric acid 8 Water 17 Tetra ethyl ureau 5.3

Solution 3 Sodium silicate 42 .5\B,. (ratio 1:3.25)

NazSiOz 10.1 Water 17.9

. acid sol containing a soluble aluminum compound and a urea.

- 3. A tanning composition comprising a silicic acid sol containing a soluble aluminum compound and urea. l

, 4. A tanning composition comprising a silicic acid sol containing aluminum sulfate and urea, there being about two mols of urea for each mol of SiOz and there being about 0.3'mol of aluminum sulfate for each mol of SiOz.

5. In a tanning process the step comprising subjecting a skin to a tanning solution comprising a silicic acid sol and an amide.

6. In a tanning process the step comprising subjecting a skin to a tanning solution comprising a silicic acid sol and a urea.

'7. In a tanning process the step comprising subjecting a skin to a tanning solution comprising a silicic acid sol and urea.

8. In a tanning process the step comprising subjecting a skin to a tanning solution comprising a silicic acid sol, a soluble aluminum compound, and a urea.

9. In a process for the preparation of a silicic acid tanning composition, the step comprising admixing an amide with a silicic acid sol.

10. In a process for the preparation of a silicic acid tanning composition, the step comprising preparing a silicic acid sol in the presence of an amide.

11. In a process for the preparation of a silicic acid tanning composition the step comprising admixing a soluble silicate with an acidic material which contains a urea.

- JOSEPH S. KIRK. 

